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  • Writer's pictureCooper Zikan

Basic Entomology 101


Understanding the insects that trout eat provides numerous advantages to any angler. There are several important aspects to entomology. These include Hatch Time, Identifying factors, aquatic habitat, and temperature. Matching the hatch will often be the most effective way to shake hands with trout. I will also point out that matching the hatch is less important in different situations. I will talk about the five main categories of insects that you will encounter on the water and provide you with the important details I mentioned above. I will go over the four main insect groups that an angler will run into when fishing most bodies of water. These include Caddis, Mayflies, Stoneflies, and Midges. I will also mention a few key species from each group that impact fisheries around the entire United States.




  Caddis are a common type of insect that hatch on most rivers in the country. They are often characterized by their tent-shaped wings and massive hatches.  The three stages of the caddis hatch that are most relevant to us are the caddis larvae, caddis pupae, and caddis adult.



The Larvae is the first main stage of the caddis. Caddis Larvae bodies are often either a brown or green color with a black or dark brown head. The Larvae will often live in cases made with river gravel and other materials to help protect them from predators.




The pupa stage is when the caddis larvae leave their shells and ascend the water column towards the surface. While ascending towards the surface, the caddis is more vulnerable to hungry fish. This is when caddis nymphs are often at their most effective. The fish will key in on the emerging caddis patterns for easy meals in both lakes and streams. The pupa has the foundation of wings and begins to resemble the shape of the adult creation.




The adult stage occurs once the caddis pupae make it to the surface of the water. From here, the wings take full shape and the adults will fly away. The adults will often fly around for close to a month until they are ready to lay their eggs. Once the caddis are ready to lay their eggs, that's when you will see the massive caddis hatches on the river take hold. This is when dry fly fishing will be the most effective way to catch trout.


Common Caddis Species


Tan Caddis:

Tan caddis is one of the more common caddis species that hatch on western waterways. This caddis will hatch generally between mid-June through August. When fishing subsurface, smaller pupae work extremely well in triggering fish to bite. The tried and true elk hair caddis do a fantastic job for an adult caddis imitation.




Mother’s Day Caddis


Fishing the Mother’s Day caddis hatch is one of the most popular hatches to fish. As the name indicates, this caddis hatches during May. This hatch tends to be rather hit or miss, with several factors playing a large role. The largest variables are the water temperature and level. If the water temp is below 50 degrees, these bugs won't hatch as prolifically. Stable flows are also crucial to effectively fish. May and June are when the water levels fluctuate the most. All it takes is one big rain to ruin the parade.



Mayflies are a type of insect that inhabits every waterway in the United States. The two species I will go into more detail with are Blue Wing Olives and Pale Morning Duns. Mayflies are characterized by their long tails and tall wings. There are three main parts of the Mayfly life cycle that are relevant to fly-fishing. These stages are nymph, emerger, and adults.


Once the eggs hatch, the nymphs will live along the bottom and feed on moss and other plant life both dead and alive. The nymphs can be displaced easily through water fluctuations, abnormal temperatures, and other factors. Mayflies can stay nymphs for as short as two weeks up to two years.


The emergent stage is when the nymphs start to form wings and slowly move up towards the surface. Similarly, to Caddis, these emergers will become more vulnerable to feeding trout. Often you will find trout-eating bugs 6 inches to a foot of the surface. These mayflies will either be emerging insects or spent adults.  


A dun is the proper name for a fully mature mayfly. Adult mayflies will typically only live for 24 hours. During this time, the females will land on the water to lay eggs, which allows trout to eat them. The best hatches are usually during warm overcast days. Light rain hasn't affected the hatches much in my experience.

Common Mayfly species:



Blue-winged olives are a common mayfly that typically hatch during the transitional seasons of fall and spring. BWO nymphs are available to trout to eat year-round. True to their name, BWO mayflies have olive-green bodies with a gray thorax. BWOs will typically hatch on cloudy days when the weather is ideally warmer with minimal wind. Days like these can result in dynamite fishing with both nymphs and dries. Usually BWO’s range from a size 14-20.



Pale Morning Duns are a mayfly that typically hatch during July and August. These mayflies have a pale-yellow color and can vary in size from 14-18 in most cases. These bugs will hatch alongside other insects including the golden stoneflies and the yellow sallies. Heading to the river without PMDs during the summer is never a wise choice.



Stoneflies are a very prevalent insect in the western United States. Stoneflies require clean water with minimal pollution to live. Stoneflies are large insects that provide trout with ample protein during all months of the year. Stoneflies are most often found in freestone streams, but can also live in tailwaters with more rocky bottoms. Popular types of stoneflies include Golden Stoneflies, Skwala Stoneflies, Salmonflies, yellow sallies, black stoneflies, and many more. While many types exist, I will go into the most detail regarding 4 types of stoneflies. Stoneflies have two different stages, which are adult and nymph.


The nymphs will live on the bottom for an undetermined period. This period is usually between 1-3 years. The nymphs can only live in moving water. Once they are ready to hatch, the nymphs will crawl along the bottom towards the banks.



The adults typically hatch on the river banks and the trees/brush near the water. Once they shed their exoskeleton, they can be seen flying around for a few weeks. When the stoneflies are ready to lay their eggs, they will deposit their eggs in the river. This makes them susceptible to feeding trout. Because of the protein surplus these insects offer, trout will key into these bugs year-round in both the nymph and adult versions.


Skwala Stonefly

Skwala stoneflies hatch between March and April. These stoneflies are one of the few that hatch during early spring. These stoneflies hatch only on select rivers and can make for some fantastic early fishing opportunities. Anytime the weather gets above 40 degrees, you can count on seeing these stoneflies flying around. Any stonefly pattern with a combination of dark olive and black will catch fish.



Salmonflies typically hatch during June when the highest water periods in the year occur. Once the high water flows start to drop, that is usually when the salmonfly hatch peaks. Salmonflies can get up to three inches long which makes them the largest of the stonefly species. Because of their size, Salmonflies find themselves the favorite target of trout during this time of year.


Golden Stoneflies

Once the salmonflies are finished hatching, the golden stoneflies soon follow. Golden Stones are slightly smaller than salmonflies, but tend to last longer and are more consistent throughout. Usually, golden stoneflies start to hatch once the runoff in the early summer subsides. Late June to late July you can find goldens flying around. Early July is normally the peak hours.


Yellow Sallies

While still stoneflies, these insects are significantly smaller than most other stonefly species. These are around a size 14-16, instead of the 8-10 that golden stoneflies normally are. July is normally the best time of year to get into consistent yellow sally hatches. Some of the best fishing can happen when yellow sallies and golden stones are both flying around. A key identifier of a yellow sally is that the end of their abdomen is red.


Midges are insects that hatch at all times of the year. Even during the below-freezing temperatures, you can find midges hatching on the water. As winter continues to take its hold, midges will be one of the top ways to catch fish consistently. Various colors and sizes of midges exist, with many of them being on the smaller side of the spectrum. Midge larvae look like little worms, which can take numerous colors. Green, Black, Tan, and Red are common colors that midges have. Fishing drowned adult midges is an effective way to catch more fish. Midges have three main stages of development, which are larvae, pupa, and adult.


Midge Larvae:

Midge larvae are little worms that usually live in mud, or on the bottom. They migrate for food and are vulnerable to hungry trout. Midge larvae are always present in most waterways and can be one of the most consistent ways to catch fish. These larvae can vary in size, but usually, the larvae are smaller in rivers and larger in lakes.


Midge Pupa:

Midge pupa are very similar to larvae, except they begin to grow their wings. The pupa will rise towards the surface from the bottom. Similar to other emerging insects, the pupa will be vulnerable to trout. Fishing pupa can be effective when fished around a foot or so under the surface.


Midge Adult:

Adult midges look essentially like a black mosquito that doesn't bite. They often fly around in clusters where trout can be seen eating multiple midges per sip. These hatches can happen anytime, but on winter days midge hatches can become prevalent. Midges do hatch year-round, so you never truly know when midges will be the optimal pattern to throw.


Whatever rivers you frequent make sure you know what hatches there, and the rough period that they do hatch. Every river is different from one another. Weather patterns are just as important as well. Timing your trips based on the weather will help you catch more fish. At the same time, you never know when you could catch a trophy. Due diligence is rewarded in the sport of fly fishing.

Tight Lines and Heavy Nets,

Cooper ZIkan

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